There is a strange Halocha regarding Tzoraas: If tzoraas partially covers ones body they are tamei. But if the Nega has spread and now covers their entire body they are Tahor. What’s the reason for this seemingly strange Halacha? A person who is covered from head to toe with tzoraas should be the most tamei. No? The meforshim explain that Hashem created Tumah to challenge a person to greater heights. It is the weapon of the Yetzer Hora who seeks to suck the good out of a person. A nega is a spiritual blemish. When the Yetzer Hora sees this, he sees a weak victim and he sees great opportunity to further the damage. He wants to take what is left of this person’s good and turn it
into bad. Therefore this person becomes tamei from the attachment of the forces of evil and impurity to his body. However if a person has a Nega covering his entire body, this means that he is completely bad and bereft of any goodness within him. In this case the forces of tumah have no work and no role to play in this person. He therefore is Tahor. This could be why people who hit rock bottom are capable of suddenly making a change for the good. The battle is over and the Yetzer Hora has gone. From there the only way is up.